1. Field of the Invention
This invention relates to the removal of deleterious materials from the waste gas produced during the oxidation of polymethylbenzene to an aromatic polycarboxylic acid. More particularly, this invention relates to a method for removing by-product methyl acetate and unreacted polymethylbenzene from such waste gas.
2. Description of the Prior Art
The liquid-phase oxidation of a polymethylbenzene, such as paraxylene, to its corresponding polycarboxylic acid, i.e., terephthalic acid, is an important commercial process. A typical solvent used in this process is acetic acid. During the process, methyl acetate is produced as a by-product, the result of oxidative degradation of the acetic acid solvent. Methyl acetate is volatile and is lost usually from the process through waste gas vents.
The recovery of methyl acetate, a useful material, is desirable. It can be converted into acetic acid and methanol by hydrolysis to furnish additional acetic acid solvent. Methyl acetate can also be marketed as a fine chemical. It is a fast-evaporating solvent, is used in lacquers, in paint and varnish removers, in the perfume industry, in the manufacture of dyestuffs, and in the manufacture of cellulosic adhesives.
Methyl acetate is present in the waste gas streams produced by the liquid-phase oxidation processes for making polycarboxylic acids in very low concentrations. Consequently, the recovery of this methyl acetate is rather difficult. Recovery methods involving condensation, scrubbing, or distillation require excessive amounts of energy and/or equipment to be made efficient.
It is known that methyl acetate can be adsorbed on activated carbon and can be desorbed from the carbon. Hori et al., J. UOEH, 9(1):9-18 (1987), estimated the breakthrough time on activated carbon for multicomponent organic solvent vapors, one solvent of which was methyl acetate. Sanchez et al., INDUST. ENG. CHEM. RES., 1987, 26(9), 1880-1887, disclosed a method for adsorbing carboxylic acids on carbon and esterifying the adsorbed acids, followed by desorbing the esters. Subbotin et al., KHIM. PROM-ST., (9), 661-662 (1974), studied the dynamics of adsorbing vapor-air mixtures containing methanol and methyl acetate on activated carbon. In Japanese patent application 35194, laid open to public inspection Mar. 15, 1979, there is disclosed the adsorption of methyl acetate, as well as other ester solvents, on active carbon and the effect of repeated regeneration and repeated adsorption on the active carbon. The breakdown time is shortened after repeated adsorption of ester solvent and steam regeneration. In Japanese patent application 6924, laid open to public inspection on Jan. 14, 1984, there is disclosed a process for the recovery of organic solvents, such as methyl acetate, which process involves adsorption of the solvent, irradiating the adsorption agent, which can be active carbon, with electromagnetic microwaves after excluding the air in the chamber, withdrawing the organic solvent vapor, and cooling the adsorption agent.
Numerous patents directed to processes for the partial oxidation of polymethylbenzenes to their corresponding aromatic polycarboxylic acids. Examples of such patents are U.S. Pat. Nos. 2,833,816, 3,089,906, and 3,089,907, issued to Saffer et al., and U.S. Pat. No. 4,053,506, issued to Park et al.
None of the prior art discloses the recovery of methyl acetate, as well as unreacted polymethylbenzenes, from the waste gas streams produced during the oxidation of polymethylbenzenes to aromatic polycarboxylic acids.
Now there have been developed an improved method for the recovery of methyl acetate from the waste gas produced during the liquid-phase oxidation of a polymethylbenzene to an aromatic polycarboxylic acid and an improved process for the liquid-phase oxidation of a polymethylbenzene to an aromatic polycarboxylic acid.